The effective cooling of transformers and inductors, particularly in aerospace applications where size and weight are important, has always been a problem. On one hand, it is important to keep the size and weight of the transformer and inductors at a minimum. On the other hand, the wire temperature must be kept at a sufficiently cool temperature to guarantee safe and reliable operation. These considerations often work at cross purposes. One way in which the prior art has attempted to deal with the problem has been by way of direct cooling, the method currently being used. Direct cooling applies to cooling medium so as to directly contact the wires and/or the iron core. In some applications, direct cooling is unacceptable.
For example, U.S. Pat. No. 2,577,825 shows a transformer in which a segment of metallic tubing is welded to the outer edges of the laminated core to utilize direct heat conduction for substantially reducing the transformer core temperature during the transformer operation. The core cooling unit is connected with the windings of the transformer so that the same coolant source may be used. With such an arrangement, the transformer can be arranged for series cooling or parallel cooling. In either type of cooling, however, this transformer is designed so that a coolant flows within the primary and secondary turns, i.e. direct contact. It will be appreciated that such an arrangement requires a much larger and heavier transformer as well as a more complex arrangement for connecting the coils to the coolant source.
U.S. Pat. No. 2,579,522 shows another transformer in which coolant connections are also provided to allow for circulation through the primary conductor windings either in series through the entire coil or in parallel.
U.S. Pat. No. 3,144,627, shows a core cooling arrangement in a stepdown transformer designed in an effort to minimize eddy currents and to efficiently dissipate heat generated in the core which is laminated and comprised of two identical E-shaped sections joined by a pair of bolts. Each of the sections has a water passage which passes centrally through its side to the center where it makes a right angle turn and passes out through the middle of the center leg. An air gap is formed between the adjacent surfaces of the end sections. Such an arrangement embeds the heat exchanger in the transformer core for cooling. However, we have found that most of the heat generated by the transformer is generated in the coils. Consequently, the arrangement shown in this patent does not provide the most efficient cooling for the transformer.
U.S. Pat. No. 3,151,304, discloses another structure which attempts to prevent the flow of eddy currents in the conductors. Two adjacent layers in a pancake coil are separated and a passageway is provided within the pancake coil for the cooling medium. However, this structure again uses direct cooling which, as noted above, is unacceptable in certain applications.
U.S. Pat. No. 3,551,863, illustrates a transformer in which heat exchange is intended to be facilitated by reducing the portion of the resistance to the flow of the heat which appears between the surface of the active parts of the transformer and cooling medium externally to reduce the temperature gradient of these active parts which are above the temperature of the cooling medium. To this end, a surface heat dissipator is connected to an exposed surface of the winding or the like of the transformer. The dissipators comprise at least one sheet of highly heat conductive material, and the heat is transferred from the heat generating active part to the stream of cooling medium through the heat dissipator. Although this arrangement is shown primarily used with air cooled transformers designed for operation with natural convection and alternative forced air cooling, it is also suggested that the dissipators can be used with a liquid cooled transformer. However, this conventional arrangement does not utilize direct contact between the surface of the heat exchanger and the coils where most of the heat is generated.
U.S. Pat. No. 4,352,078, shows an electrical inductive apparatus, such as a transformer, having pancake coils, an outer bag surrounding a foil-coated core and a dielectric fluid coolant introduced into the outer bag. The bag contains a core, static plate and insulating coating which is able to conform to any forces applied to it internally or externally and thereby distribute the dielectric coolant over the stacked pancake coils from which the coolant flows by gravity. Such an arrangement does not permit, however, the heat which is generated in the high voltage and low voltage coils to be in direct contact with the surface of the flexible bag.
U.S. Pat. No. 4,482,879 shows a transformer core cooling arrangement in which a thin, flat molded frame having a contour corresponding to the core laminations of a transformer is interleaved between an adjacent pair of the core laminations in liquid-type relationship thereto. This arrangement forms a plurality of internal passageways within the core for the passage of a liquid cooling medium in direct contact with the core laminations 22 to effect core cooling during transformer operation. Again, such an arrangement is not designed to maximize heat transfer from the area in which most heat is generated, namely the coils.
U.S. Pat. No. 4,491,817, shows a sheet wound transformer in which sheet conductors are wound into coils with an insulating sheet interposed between adjacent turns. Arcuate cooling panels are provided in the coils to maintain the coolant circuit completely separated from the insulating gas such as SF.sub.6. To avoid high current density and a local temperature rise at the upper and lower end portions of the sheet-wound coils because of eddy currents flowing in the conductors, ribs in the cooling panels have inlet ends and outlet ends disposed obliquely to supply a larger part of the coolant through the upper and lower portions of the panels. Although such an arrangement is intended to reduce the size and weight of the transformer, it is limited to a sheet wound transformer in which the cooling panels are formed in arcuate or cylindrical shapes.
U.S. Pat. No. 4,739,825, shows another conventional core cooling arrangement for a liquid cooled transformer. However, as previously noted, it is the coils, not the core, which requires the most effective heat exchange.